A three-way catalytic converter is disposed in an exhaust system of an engine to purify an exhaust gas. To provide the catalytic converter with converting efficiency, it is usual to carry out feedback control having the intake air-fuel mixture to the engine maintain a theoretical air-fuel ratio.
The air-fuel ratio feedback control employs an air-fuel ratio sensor (an oxygen sensor) for detecting an air-fuel ratio according to the concentration of oxygen contained in an exhaust gas. To ensure good response from the air-fuel ratio sensor, the sensor is disposed at, for example, a collecting portion of an exhaust manifold in the vicinity of a combustion chamber. The sensor detects the concentration of oxygen contained in the exhaust, and according to the detected concentration, it is determined whether an actual air-fuel ratio is rich or lean with respect to a theoretical air-fuel ratio. According to the rich or lean determination, the feedback control adjusts the supply of a fuel to the engine so that the actual air-fuel ratio will agree with the theoretical air-fuel ratio.
Since the air-fuel ratio sensor is disposed close to the combustion chamber in the exhaust system, the sensor is exposed to high-temperature exhaust, which may thermally deteriorate the characteristics of the sensor. When the oxygen sensor is disposed at the collecting portion of the exhaust manifold, where exhausts from respective cylinders are not sufficiently mixed together yet, the oxygen sensor does not accurately detect a mean air-fuel ratio of all cylinders. Accordingly, the conventional air-fuel ratio feedback control may ensure responsiveness but hardly achieves accurate stabilized air-fuel ratio control.
To solve this problem, it has been proposed to arrange another oxygen sensor on the down stream side of the catalytic converter in addition to the one disposed on the upstream side thereof, and carry out air-fuel ratio feedback control according to values detected by the two oxygen sensors (Japanese Unexamined Patent Publication No. 58-48756).
Although the downstream air-fuel ratio sensor is not advantageous in terms of responsiveness due to its distance from the combustion chamber, it is less affected by heat and toxic components of an exhaust gas on the downstream side of the catalytic converter, and therefore, it can detect a mean air-fuel ratio of all cylinders because it receives a well mixed exhaust. As a result, the downstream air-fuel ratio sensor provides more accurate and stabilized detections compared with those provided by the upstream air-fuel ratio sensor.
Values detected by the two air-fuel ratio sensors are used to set two different air-fuel ratio feedback correction coefficients, which may be combined together and used. Alternatively, a value detected by the downstream air-fuel ratio sensor is used to correct a control quantity (a proportional portion or an integral portion) applied to an air-fuel ratio feedback correction coefficient set by the upstream air-fuel ratio sensor, or correct a comparison voltage used for an output voltage of the upstream air-fuel ratio sensor, or correct a delay time occurring when using a detected result of the upstream air-fuel ratio sensor for actual control. In this way, fluctuations of the output characteristics of the upstream air-fuel ratio sensor are compensated for with the downstream air-fuel ratio sensor, to accurately achieve air-fuel ratio feedback control.
When using the two air-fuel ratio sensors (usually, oxygen sensors) for controlling an air-fuel ratio, a required level of air-fuel correction during the feedback control sometimes greatly differs from that required during non-feedback control (during an open loop). In particular, when the feedback control is started after the non-feedback control, the following problem may occur:
Air-fuel ratio detection by the downstream air-fuel ratio sensor is delayed from that by the upstream air-fuel ratio sensor. If an air-fuel ratio correction control speed by the downstream air-fuel ratio sensor is set to be substantially equal to that of the upstream air-fuel ratio sensor, a large overshoot may occur in the control. To prevent this, the air-fuel ratio control speed of the downstream air-fuel ratio sensor is slowed compared with that of the upstream air-fuel ratio sensor.
As a result it takes time for an air-fuel ratio correction quantity (for example, a correction quantity for a proportional portion of proportional-plus-integral control carried out based on an air-fuel ratio feedback correction quantity derived from the upstream air-fuel ratio sensor) controlled based on the downstream air-fuel ratio sensor to reach a required value. This extends the time required for attaining a target air-fuel ratio, and deteriorates fuel consumption, operability, and the quality of an exhaust gas.
When an operating condition of the engine is shifted from one operation region to a different one during the air-fuel ratio feedback control, an actual air-fuel ratio may greatly deviate from a target air-fuel ratio due to the difference of required air-fuel correction levels between the operation regions. This also deteriorates fuel consumption, operability, and the quality of an exhaust gas.
To solve the problem, it has been proposed to continuously calculate and store mean air-fuel ratio correction quantities as learned correction quantities according to the downstream air-fuel ratio sensor. The learned correction quantities are used with the air-fuel ratio correction quantity to always stably control the air-fuel ratio (Japanese Unexamined Patent Publication No. 63-97851).
Since the air-fuel ratio correction control speed of the downstream air-fuel ratio sensor is set to be relatively low to prevent overshooting, an air-fuel ratio correction quantity for each operation region according to the downstream sensor will not be quickly learned. In addition, the required air-fuel ratio correction quantity greatly changes depending on operating conditions, so that it is preferable to divide a learning region into small regions, to secure learning accuracy. When the region is divided into small regions, however, a learning time for each small region becomes shorter, and each small region seldom corresponds to an acutal operating condition. This prevents a progress of learning.
The conventional technique, therefore, does not realize the two objectives of promoting learning and improving learning accuracy. As a compromise, one conventional technique employs relatively large divided operation regions for storing learned correction quantities, respectively. This, however, deteriorates operability because it deteriorates the quality of an exhaust and fluctuates air-fuel ratio.
To solve the problem, there has been proposed a method of and an apparatus for controlling the air-fuel ratio of an internal combustion engine by simultaneously caring out and balancing collective learning in a wide operation region for improving learning speed, and regional learning in each divided operation region for maintaining learning accuracy. This technique improves the progress and accuracy of air-fuel ratio control learning carried out according to the downstream air-fuel ratio sensor.
This air-fuel ratio control technique with double learning, however, has the following problem to be solved:
When the collective learning for a wide operation region and regional learning for each divided operation region are simultaneously carried out, and when a learned correction quantity for the wide operation range is updated through addition, all divided operation regions in the wide operation region are controlled according to the updated learned correction quantity. It is necessary, therefore, to update a learned correction quantity of each of the divided operation regions by subtracting the updated portion therefrom, to prevent an excessive correction to be made according to the learned correction quantity.
Since the learned quantities of all divided regions are updated through subtraction when the learned collective quantity is updated through addition, a learned quantity stored in any divided operation region whose learning opportunity is very small will not converge, because the learned quantity is updated through subtraction by the learned collective correction quantity before convergence. As a result, the region of small learning opportunity is further delayed to converge. The simultaneous collective and regional learning for improving learning speed and accuracy is not so effective if carried out with the downstream air-fuel ratio sensor having a detection response delay.
To solve these problems, an object of the invention is to provide a method of and an apparatus for controlling the air-fuel ratio of an internal combustion engine, by simultaneously carrying out, in a balanced manner, collective learning in a wide operation region for increasing learning speed and regional learning in divided operation regions for maintaining learning accuracy, thereby improving the accuracy and speed of air-fuel ratio learning control carried out according to a downstream air-fuel ratio sensor.
For this air-fuel ratio learning control with the simultaneous collective and regional learning carried out with the downstream air-fuel ratio sensor, another object of the invention is to quickly converge a value stored in any divided operation region that is seldom learned in the regional learning.
Still another object of the invention is to surely and simply determine any divided operation region that is seldom learned in the regional learning.